1. Technical Field
This invention is directed to novel equilibrium modifiers for use with oxime extractant reagents in water-immiscible hydrocarbon solvent formulations for the recovery of copper from acidic leach solutions containing copper values and other metal ions.
2. Background and Related Art
The starting material for large-scale solvent exaction processing of copper is an aqueous leach solution—usually a sulfuric acid solution, but it may also be a basic aqueous solution when ammonia is the leaching agent—that is distributed over mined ore containing a mixture of metals in addition to copper, dissolving salts of copper and other metals as the leach solution trickles through that ore.
The aqueous leach solution with its resulting mixture of metal values is then mixed in mixer tanks with a water-immiscible liquid hydrocarbon solvent (e.g., kerosene) containing one or more extractant chemicals (e.g., oximes), possibly including one or more equilibrium modifiers, that selectively forms a metal-extractant complex or chelate with the copper ions/values in preference to ions of other metals, in a step called the extraction or loading stage of the solvent extraction process. The outlet of such tanks continuously feeds to a large settling tank, where the organic solvent (organic phase), now containing the copper-extractant complex in solution, is separated from the copper-depleted aqueous solution (aqueous phase) in a phase separation stage, a step that may be complicated by the presence of such one or more equilibrium modifiers, which hinder phase separation and/or may cause the build-up of crud at the boundary of the phases. At the higher concentrations of modifier(s) in the extractant formulation, the modifier contributes significantly to the viscosity of the overall reagent formulation, and therefore, being able to use less modifier component is an advantage simply because the overall viscosity of the organic phase is also lower—a particularly important advantage in the phase separation stages.
After extraction and phase separation, the metal-depleted aqueous feedstock (raffinate) is either discharged or recirculated to the ore body for further leaching. The loaded organic phase, now pregnant with the dissolved copper-extractant complex is fed, possibly after a washing stage to facilitate removal of undesired amounts of iron and other metal ions, to a stripping stage, comprising another set of mixer tanks, where it is mixed with an aqueous sulfuric acid strip solution. This strip solution breaks apart the copper-extractant complex and permits the extracted copper to pass to another settler tank for another phase separation, where, again, equilibrium modifiers may cause inefficient phase separation and undesired entrainment of the organic phase in the resulting strip aqueous phase. On the other hand, however, adding a limited quantity of one or more equilibrium modifiers to the extractant formulation shifts the equilibria in such a manner that one can efficiently strip higher amounts of copper from the extractant using conventional stripping solutions, generating a more copper-rich electrolyte, well suited for the electrodeposition of high purity copper.
From the stripping settler MA, the “regenerated” organic phase, effectively stripped of its metal values, is recycled to the extraction mixers to begin extraction again, and the copper-rich strip aqueous phase is customarily fed to an electrowinning tankhouse, where the copper metal values are collected on plates by a process of electrodeposition. Then, after electrowinning to harvest the copper values from the aqueous solution, the copper-depleted solution, known as spent electrolyte, is returned to the stripping mixers to begin stripping again.
Modifiers of extraction and stripping equilibria are frequently incorporated in the commercial reagent formulations, when such formulations include the so-called “strong” extractants, e.g., the aldoximes. Such extractants are capable of forming a very stable complex association with copper at quite low pH's and, consequently, require the use of very highly acidic aqueous stripping solutions in order to effect the breakdown of the copper-extractant complex. The resultant copper-rich aqueous strip solution, however, is not suitable for the electrowinning of high purity copper metal due to the high acid concentration and the relatively low copper concentration. The solubility of copper sulfate is depressed at high sulfuric acid concentrations.
The use of modified aldoximes (i.e., an aldoxime extractant plus an equilibrium modifier) to extract copper from aqueous acidic sulfate solutions is well known. ICI introduced P5100, 5-nonylsalicylaldoxime (NSO) modified by nonylphenol (NP), to the industry in the early 80's, then Henkel introduced LIX 622, a mixture of isotridecyl alcohol (TDA) with 5-dodecylsalicylaldoxime, and LIX 622N, TDA in combination with NSO. U.S. Pat. Nos. 4,978,788; 5,176,843; 5,281,336; 6,113,804; and 6,277,300 (all, Dalton et al) describe formulations based on the use of highly-branched alcohols and esters, such as 2,2,4-trimethyl-1,3-pentanediol di-isobutyrate (TXIB), as modifiers. U.S. Pat. No. 6,177,055 B1 (Virnig et al) discloses the use of linear esters, such as di-n-butyl adipate (DBA), as modifiers. U.S. Pat. No. 6,231,784 B1 ('784, Virnig et al) discloses a very broad range of chemical classes, including simple carboxylic acid esters, oximes, nitriles, ketones, amides (carboxamides, sulfonamides and/or phosphoramides), carbonates, carbamates, sulfoxides, ureas, phosphine oxides, alcohols, ester ethers, polyethers and mixtures thereof, that can be used in combination with the NSO aldoxime to formulate copper solvent extraction reagents.
Two articles providing analysis of the effects of modified oxime exaction reagents on the extraction and recovery process are: “Discussion of the Physiochemical Effects of Modifiers on the Extraction Properties of hydroxyamines; A Review”, A. M. Sastre and J. Szymanowski, Solvent Extraction and Ion Exchange, Vol. 22(5), pp 737-759 (2004); and “Equilibrium Modifiers in Copper Solvent Exaction Reagents—Friend or Foe?”, G. Kordosky and M. Viring, Proceedings of Hydromet 2003, TMS, 2003.
The currently-used modifiers require fairly high modifier concentrations relative to the one or more aldoxime extractants in order to achieve the desired modifying effect. This increases the overall cost of the extractant/modifier reagent and increases the potential adverse effects of modifiers, such as increased viscosity in the organic phase and increased density of the organic phase, both of which contribute to poor phase separation and/or crud generation in the mixer/settlers. Accordingly, it was an object of the present invention to find equilibrium modifiers which deliver effective levels of modification at lower molar ratios of modifier-to-aldoxime extraction reagent(s).
Modifiers function by hydrogen bonding with the oxime. Thus, increasing the steric bulkiness around the functional group of the modifier(s) would be expected to make the functional group less available to form the necessary hydrogen bonding. Indeed, if one increases the steric bulk by introducing additional branching on both sides of the ether functionality, one will depress the overall effectiveness of the ether as a thermodynamic modifier. For these reasons, it was completely surprising that increasing steric bulkiness close to only one side of an ether oxygen atom had the opposite effect that is, of a lower ether modifier(s)-to-extractant reagent(s) ratio necessary to achieve the same level of modification, even as compared to ethers with no steric hindrance on either side of the ether oxygen functionality.